The Missing Piece / soap

After being used for a period of time, a soap bar will become too small to hold firmly.
Thus, it will either break or loose easily. This design attempts to solve this problem.

“The missing piece” looks just like a normal soap bar but has a shallow concave on one side. Before starting to use a new soap bar, people can fill the shallow concave with the old one to complete the soap bar. As time goes by, old soap bar will melt and combine with new one completely.
Therefore, nothing will be wasted.

Till the soap bar becoming a small piece again, it's time to complete another missing piece!